Carrier Synchronization

Traditionally, carrier synchronization (sync) techniques have been developed assuming that the modulation format and signal constellation characteristics are known a priori. By modulation format we mean that the modulation index is chosen so that either the carrier is fully suppressed or a residual carrier component remains. By constellation characteristics we refer to the shape of the constellation, e.g., a circle for M -ary phase-shift keying (M -PSK) or a square for quadrature amplitude modulation (QAM), and its size in terms of the number of signal points it contains. Aside from knowing the modulation index and signal constellation structure, it is also customary to have knowledge of the data rate and type (e.g., non-return to zero (NRZ) versus Manchester code) since the true optimum design of the loop depends on this information. In autonomous radio operation, the most optimistic situation would be that the receiver contain a carrier synchronization structure that is capable of tracking the carrier phase independently of the above-mentioned considerations. Unfortunately, this is not completely possible since, for example, a squaring loop (or equivalently a binary phase-shift keying (BPSK) Costas loop) cannot track a quadrature phase-shift keying (QPSK) modulation and likewise a 4th power loop (or equivalently a QPSK Costas loop, sometimes referred to as an in-phase– quadrature (I-Q) loop) cannot properly track a BPSK signal. Nevertheless, while in principle each carrier synchronization loop developed for a given modulation format, constellation, and data rate/type has certain unique characteristics, they do share a number of similarities, e.g., a common front-end demodulator